The whole point of
breathing is to get gas exchange to occur between the blood and the air, so that oxygen
can get into our tissues. Another function of breathing is to release our gas waste
product: carbon dioxide. All of this gas exchange occurs in the alveoli of the
lungs. That's where we are headed on this page!

This figure from your book (Figure 19.16) depicts what it might look like if you cut through an
alveolus and looked in from the cut edge. You would see the simple squamous
epithelium that lines the alveolus... that's for sure. Can you see it in this
picture? The word "Air" is on top of one squamous cell, just to the right
of its nucleus. Another squamous cell was cut right through its nucleus, which can
be seen under the oxygen arrow. Sporadically interspersed with the squamous cells
are the occasional surfactant-secreting cells.

Another feature you would see in a cut alveolus is the capillary bed
that runs around the outside of the entire structure. The capillary bed is from
arteriole to venule, and is depicted as running from top left, down and to the right, and
reaching the venule at the top right. Remember, pulmonary arterioles contain
deoxyhemoglobin and are color coded in blue, while pulmonary venules contain oxyhemoglobin
and are color coded in red.

Consider what materials the bottom oxygen molecule (next to the
surfactant-secreting cell) would encounter on its trip from the alveolus to the blood.
The oxygen atom would first have to travel through the alveolar epithelial cell.
Then it would
have to pass through any materials connecting the alveolar epithelium to the
capillary. And finally it would have to pass through the capillary wall; the
capillary wall is only one, simple squamous cell (the endothelial cell) thick. I
have tried to show this for you in this little drawing.

It is certainly not a lot of tissue that the gases have to travel
through. Only 2 cells thick (with a little extracellular material in between them,
just the basement membrane). This 2-cell-thick-material is called the respiratory
membrane.

Now lets look at the actual exchange. We can use this figure
(Figure 19.34) from your book to help...

When blood first arrives at the pulmonary capillary at its arteriole
end, the partial pressures of carbon dioxide and oxygen are:
PCO2 = 45 mm Hg
PO2 = 40 mm Hg
These partial pressures need a bit more explanation. You see,
there's a lot of carbon dioxide in blood since all the waste carbon dioxide from the
tissues of the body is in it. You had seen that the partial pressure of carbon
dioxide in the atmosphere is much tinier than this. Next, the partial pressure of
oxygen is only 40 mm Hg in this blood; that is because much of the oxygen that we had in
the blood left it while travelling all over the body. You saw that the partial
pressure of oxygen in the atmosphere was 160 mm Hg, which is much higher than 40 mm Hg!

What are the partial pressures of these gases in the alveolar space?
They are NOT the same as the atmospheric partial pressures. Remember the
terms "functional residual capacity" and "residual volume" from the volumes and capacities page? These are the terms that
describe the air that doesn't normally leave the respiratory tract. Because the
amount of air that never leaves the respiratory tract is so large (over a liter!), the
spaces in the respiratory tract can never totally empty out and be completely replaced by
atmospheric air. So the alveolar air is a mixture of atmospheric air and functional
residual capacity air. Therefore, rather than the PCO2 being
equal to .3 mm Hg, the PCO2 = 40 mm Hg. That is a huge
difference! More than a hundred-fold difference. And, rather than the PO2
being equal to 160 mm Hg, PO2 = 104 mm Hg.

Once you understand that the partial pressures of the gases within
the alveoli are not the same as their atmospheric partial pressures, you can understand
what has to happen next.

Gas

Partial Pressure in Blood

Action

Partial Pressure in Alveolar Space

Carbon Dioxide

45 mm Hg

exit

40 mm Hg

Oxygen

40 mm Hg

enter blood

104 mm Hg

Carbon dioxide leaves
the blood along its pressure gradient, while oxygen enters the blood along its pressure
gradient. When carbon dioxide leaves the blood, it enters the alveolar space and is
exhaled into our environment.